The present invention relates to a liquid acid catalyzed alkylation unit. More specifically, the present invention relates to the in situ measurement and the control of a low inventory liquid acid catalyzed alkylation unit.
Alkylation is a reaction in which an alkyl group is added to an organic molecule. Thus, an isoparaffin can be reacted with an olefin to provide an isoparaffin of higher molecular weight. The process depends on the reaction of a C.sub.2 to C.sub.5 olefin with isobutane in the presence of an acidic catalyst producing an alkylate. This alkylate is a valuable blending component in the manufacture of gasolines due not only to its high octane rating but also its sensitivity to octane enhancing additives.
U.S. Pat. No. 4,795,728 discloses a hydrofluoric acid (HF) catalyzed alkylation process for producing motor fuel. The hydrofluoric acid catalyst complex contains from 0.5 to 5 weight percent of a cationic or anionic surfactant component enabling the process to be operated at an olefin acid volumetric feed ratio of greater than 1.0 while maintaining acceptable alkylate quality.
For a general discussion of sulfuric acid alkylation, see the series of three articles by L. F. Albright et. al., "Alkylation of Isobutane with C.sub.4 Olefins", 27 Ind. Eng. Chem. Res, pgs. 381-397 (1988).HF alkylation is described in further detail in the Handbook of Petroleum Refining Processes, pgs. 3-28 (1986).
Generally, in acid alkylation longer residence times for the hydrocarbon/acid contact are preferred. However, longer residence times result in reduced reactor capacity as well as increased operating costs. For a discussion of residence time see Albright, "Modern Alkylation" Oil and Gas Journal, p. 83, (Nov. 12, 1990).
Lewis acids are reducing acids having a high vapor pressure, and a propensity to flash into a cloud. Lewis acid catalyzed alkylation processes are also currently used to produce high octane blending components. Examples of Lewis acids include BF.sub.3, AlCl.sub.3 and SbF.sub.3.
Liquid acid catalyzed continuous alkylation processes generally comprise a reactor, a settler where hydrocarbon droplets are separated from the acid and a heat exchanger where the heat generated by the exothermic reaction is removed. Each vessel requires a large liquid acid catalyst inventory.
Both sulfuric acid and HF alkylation share inherent drawbacks including environmental and safety concerns and acid consumption. While catalyst complexes comprising BF.sub.3 overcome some of the safety and environmental drawbacks of sulfuric acid and HF alkylation systems, the volume and quality of BF.sub.3 alkylates have not, proven comparable to that of sulfuric or HF alkylates. Currently HF catalyzed alkylates processes are under particular safety and environmental scrutiny, because of the toxic and corrosive nature of HF.
U.S. Pat. Nos. 4,938,935 and 4,938,936 describe the danger of HF leaks. Through many safety precautions are taken to prevent leaks, massive or catastrophic leaks are feared primarily because the anhydrous acid will fume on escape crating a vapor cloud that can be spread for some distance.
It is therefore an object of the present invention to provide a method and system for reducing the liquid acid catalyst inventory in acid catalyzed continuous alkylation processes.
It is a further object to provide a method and system for improving the safety of liquid acid catalyzed continuous alkylation.
It is a further object of the present invention to provide a method and system for minimizing the risk of a sudden release of toxic material.